The invention relates to a method for the removal of gaseous organic substances from the air with an adsorbent in an adsorber, in which the adsorbent is regenerated by microwaves. It also has as an object a system for performing the process and the use of this system.
The desorption of regeneratively driven adsorption systems is customarily carried out with hot gas desorption or steam desorption technology. Since the energy input is coupled with the volume flow in this context, concentration of the desorbate is only possible to a limited extent. Particularly in hot gas desorption, the efficiency level is low since the heat capacity of air is low and heat transfer between air and the adsorbent is unfavorable. As the concentration of pollutants decreases, the energy requirement per quantity of removed pollutant increases. Thus operation of such a system in a manner that is feasible in terms of energy is only possible to a limited extent.
Due to the high dilution of the pollutants in the hot gas desorption air, disposal of the desorbent requires an additional expenditure of energy, for example by means of a burner.
Since polar molecules, in particular water, absorb microwaves, it is known to vaporize water, but also other polar organic compounds such as alcohols, that are adsorbed by an adsorbent, with a microwave generator in order to regenerate the adsorbent.
However, organic compounds that are less polar or not polar at all cannot be removed in this way. Yet the air contains a number of nonpolar or slightly polar organic pollutants that are adsorbed by the adsorbent and thus must be desorbed again during regeneration. Examples of nonpolar organic compounds in the air are alkanes or other aliphatic hydrocarbons as well as aromatic hydrocarbons such as benzene.
The object of the invention is to provide an effective, energy-efficient process for the removal of gaseous organic substances from the air.
In accordance with the invention, the adsorbed organic substances are desorbed through irradiation with microwave energy for regeneration of the adsorbent. In this process, the energy input is achieved either through direct excitation of the adsorbate (especially in the case of polar compounds, such as water, ethanol or aldehydes) and/or in that the adsorbent absorbs microwaves. In the latter case, in contrast to the desorption of polar compounds, the microwave-absorbing adsorbent is itself heated so that even nonpolar or slightly polar organic compounds are evaporated, which is to say desorbed.
Zeolites in particular have proven to be suitable as adsorbents for air cleaning, and specifically hydrophobic zeolites, which is to say zeolites with a high silicon/aluminum molar ratio. In contrast, hydrophilic zeolites with a low silicon/aluminum molar ratio preferentially adsorb water and thus have only a low adsorption capacity remaining for the organic pollutants contained in the air, hence are unsuitable for air cleaning.
The ability of a zeolitic adsorbent to be excited by microwaves is contingent upon the ion conductivity of the exchangeable cations in zeolite. The ion conductivity results from transposition of the relatively mobile cations in the zeolite lattice. Ion conductivity can be influenced by such factors as the choice of cation, the aluminum content of the lattice (modulus), the lattice type and the water content. Since the dipole character of the zeolite lattice is also dependent on the ion conductivity, the ability of a zeolitic adsorbent to be excited can be set selectively in this manner.
While the known hydrophobic zeolites are indeed suitable as adsorbents, they do not possess adequate microwave adsorption capability. As has been demonstrated, however, the microwave adsorption capacity of hydrophobic zeolites can be raised.
One possibility for doing so is to reduce the silicon/aluminum ratio of hydrophobic zeolites. For example, a commercially available hydrophobic zeolite has a silicon/aluminum ratio of approximately 100, while a commercially available hydrophilic zeolite has a silicon/aluminum ratio of approximately 1. Accordingly, the silicon/aluminum molar ratio of the zeolite should be at least 10, preferably at least 30, and less than 100, preferably no more than 90, in order to obtain zeolites that absorb microwaves on the one hand and are suitable as adsorbents on the other hand.
To set such a silicon/aluminum ratio, an ordinary commercial hydrophilic zeolite, for example, can be treated with steam or an ordinary commercial hydrophobic zeolite can be treated with silicon tetrachloride in order to dissolve out the aluminum or replace it with silicon.
If the adsorbent loaded with the pollutants has adequate microwave absorption capacity, it can then be heated in accordance with the invention directly through microwave excitation. However it is also possible in accordance with the invention to use an adsorbent that does not itself have adequate microwave absorption capacity provided that an additional solid is present in the adsorber, which itself has a high microwave adsorption capacity and thus heats the adsorbent to an adequate temperature for desorption of the organic substances. The microwave-absorbing solid can be, for example, a hydrophilic zeolite, activated charcoal, a polymer or the like.
The adsorbent can be present in a mixture with the microwave-absorbing solid. For example, the adsorber can be filled with a mixture of a (microwave-insensitive) hydrophobic zeolite and a hydrophilic zeolite as a microwave-absorbing solid. The weight ratio of the hydrophilic zeolite to the hydrophobic zeolite in such a mixture is preferably 1:10 to 1:30.
The water adsorbed from the ambient air can be excited very selectively through microwave energy. The water content of the loaded adsorber can be set through the mixture ratio of different zeolites (zeolite types, hydrophilic and hydrophobic). In this way, the energy input or the temperature level to be achieved can be set. The desorbed water displaces the nonpolar substances that cannot be excited directly by microwave energy, in a similar manner to hot steam desorption, by displacement desorption.
Through the use of microwaves, in the method in accordance with the invention the energy is quickly and very efficiently put to use for the desorption process. As a result, rapid and low-loss desorption is possible.
The frequency used for microwave desorption can be in the range from 100 MHz to 10 GHz, for example; preferably it is 2.45 GHz.
In the method in accordance with the invention, the desorption air does not represent a carrier of heat energy, for example in contrast to hot gas desorption. Therefore it can be used in relatively small quantities. Consequently, the desorbate is present in a high concentration and can therefore be fed to a catalytic oxidation process in a manner that is favorable with regard to energy.
In the method in accordance with the invention, the oxidation catalyst with which the adsorbed organic substances are oxidized can either be directly added to the adsorbent or the organic substances can be desorbed and oxidized in a catalyst that is separate from the adsorber.
The organic substances are oxidized to carbon dioxide and water by the desorption air on the oxidation catalyst. The desorption air with the oxidized organic substances is therefore preferably recirculated to the adsorber for desorption. The heat of combustion and the energy that passes from the adsorber into the desorption air by convection is thus fed back to the desorption process (heat recovery).
The oxidation catalyst can be, for example, platinum or a metal of the platinum group. For example, when oxidation of the organic substances occurs in the adsorber during regeneration of the adsorber, the zeolite or other adsorbent can contain platinum in the 0 oxidation stage (metallic clusters). Platinum or the other oxidation catalyst additionally leads to a sharp increase in the microwave absorption capacity of the adsorbent.
The method in accordance with the invention, or the system in accordance with the invention, is intended especially for cleaning of the air in spaces used by persons, especially for the cleaning of air in the interior of motor vehicles, including airplanes, and also in buildings or shelters, including for protection from contaminated outdoor air.
For preliminary cleaning of the airstream to be cleaned, a prefilter is used for particle separation. Contamination of air treatment systems with germs represents a not insignificant health hazard. At the same time, the adsorption performance is reduced by biological materials on the adsorbent. Contamination with germs is reduced efficiently through irradiation of the particle separation prefilter by microwave energy. In this context, the microwave energy for irradiating the prefilter can come from the microwave generator for irradiating the absorber, or a separate microwave generator can be employed for irradiating the prefilter.